Basically, an injector for an injection system, in particular a common-rail diesel injection system, is comprised of several parts which, as a rule, are held together by a nozzle clamping nut. In the body of the injector nozzle itself, a nozzle needle is guided in a longitudinally displaceable manner, which nozzle needle has several open spaces via which fuel is able to flow from the nozzle prechamber to the tip of the nozzle needle. As a rule, a sealing seat is provided on the tip of the nozzle needle to prevent fuel from reaching the combustion chamber when the nozzle needle is closed. The nozzle needle, on its periphery, comprises a collar on which a pressure spring is supported, which acts closingly on the nozzle needle. The nozzle needle end opposite the tip of the nozzle needle opens into a control chamber that can be powered with pressurized fuel. To this control chamber can be connected at least one inlet channel and at least one outlet channel. All of the connected channels may each comprise at least one throttling point. The pressure within the control chamber is controllable by a control valve which, in most cases, is actuated by an electromagnet. When the valve is actuated, fuel can flow out of the control chamber, thus lowering the pressure in the same. Below an adjustable control chamber pressure, the fuel pressure exerted on the sealing seat will open the nozzle needle, thus causing fuel to be injected into the combustion chamber through at least one injection hole. The flow rates through the individual channels, which are provided with throttles, determine the opening and closing speeds of the nozzle needle.
If an injector of this type is operated with highly viscous fuels—e.g. heavy oil—, it may be necessary to heat the fuel in order to achieve the required injection viscosity. It is, therefore, common, when using such fuels, to flush the injection system with a second fuel of lower viscosity—e.g. diesel oil—before stopping the engine. This helps to prevent highly viscous fuel from cooling down in the injector and affecting, or even rendering impossible, the function of the injection system during the start of the engine.
U.S. Pat. No. 5,201,341 A shows and describes an electromagnetic valve for controlling a fluid flow, as may be used in fuel injectors, in which the fuel to be heated is heated by a fluctuating magnetic field generated by the coil of an electromagnet.
DE 10100375 A1 shows and describes a method for operating a heating oil burner including an atomizing means comprising a nozzle assembly through which heating oil flows and which can be heated by electric energy, in which the heating energy for heating the heating oil is introduced by the appropriate energization of the actuator coil of a magnetic valve. In that method, heating is effected by the current fed to the actuator both during the magnetic-valve operation phase and during the warm-up phase with the magnetic valve closed.
From DE 10136049 A1, a method for heating fuel in a fuel injector comprising one or several magnetic coils has become known, wherein the magnetic coil of the injector of a fuel injector is utilized for heating the fuel. The method proposed in that document is applicable both to fuel injectors including single-coil magnetic assemblies and to fuel injectors comprising double-coil magnetic assemblies, for activating the fuel injection valves. The magnetic coil provided on the fuel injector in those cases is operated as a heating element so as to enable, on the one hand, the saving of an additional heating element and, hence, of costs and structural space, and, on the other hand, due to the arrangement of the magnetic coils within the fuel injector, the rapid heating of the injector body and, hence, the rapid heating of the fuel volume supplied from a fuel delivery installation or a high-pressure collection chamber.
From DE 4431189 A1, a method for preheating the fuel for an internal combustion engine is known, in which, in the event of cold fuel, the electric power loss of the electrical actuation is increased by the aid of an electrically operated injection valve for the fuel and its waste heat is used for preheating the fuel. By the aid of the proposed method, it is recommended, as a substitution for separate electrical heating elements in engines having electrically or electromagnetically operated injection nozzles, to feed the thermal energy for heating the fuel by artificially increasing the energy supply to the electrical or electromagnetic valve actuation of the injection valves. This may, for instance, be realized in that an electrical contact is closed during the opening of the vehicle door, which electrical contact allows electrical current to flow through the windings of injection nozzles as a function of the ambient and coolant temperatures over a defined time, or until a defined fuel temperature is reached. In doing so, it is ensured that no fuel is yet reaching injection despite those measures.
However, the method known from DE 4431189 A1 does in no way guarantee that also highly viscous fuels such as, e.g. heavy oil, will be sufficiently heated so as to induce the viscosity reduction required for injection. There is, in particular, no control as to whether the heating of the injector has actually led to the desired result, i.e., that the valve closing member is freely movable without being impeded by viscous heavy oil.